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c87df013cb
I noticed something that seemed really strange with the i386 register
numbering.
In i386-linux-tdep.h we setup I386_LINUX_ORIG_EAX_REGNUM based on
I386_PKRU_REGNUM.
However, in i386-tdep.h, enum i386_regnum ends like this:
enum i386_regnum
{
...
I386_ZMM7H_REGNUM = I386_ZMM0H_REGNUM + 7,
I386_PKRU_REGNUM,
I386_PL3_SSP_REGNUM,
I386_FSBASE_REGNUM,
I386_GSBASE_REGNUM
};
So I386_LINUX_ORIG_EAX_REGNUM will have the same value as
I386_PL3_SSP_REGNUM.
The I386_PL3_SSP_REGNUM was added in commit:
commit 63b862be76
AuthorDate: Fri Mar 29 16:38:50 2019 +0100
CommitDate: Fri Aug 29 17:02:09 2025 +0000
gdb, gdbserver: Add support of Intel shadow stack pointer register.
And before that, I386_FSBASE_REGNUM and I386_GSBASE_REGNUM were added
in commit:
commit 1163a4b7a3
AuthorDate: Tue Mar 12 13:39:02 2019 -0700
CommitDate: Tue Mar 12 13:39:02 2019 -0700
Support the fs_base and gs_base registers on i386.
So the SSP overlap is new, but the fs/gs base overlap has existed for
years, so why did it not cause any problems?
I think the explanation is that on i386, the fs/gs base are only used
for FreeBSD, all the calls to i386_target_description that pass true
for the segments argument are from fbsd files. As a result, its fine
if there's numbering overlap between these i386 registers and some
Linux specific i386 registers.
OK, but what about the new SSP (shadow stack pointer) register?
I think in this case we would see problems, if the shadow stack was
supported for i386. Here's what the docs say:
The ‘org.gnu.gdb.i386.pl3_ssp’ feature is optional. It should
describe the user mode register ‘pl3_ssp’ which has 64 bits on amd64, 32
bits on amd64 with 32-bit pointer size (X32) and 32 bits on i386.
Following the restriction of the Linux kernel, only GDB for amd64
targets makes use of this feature for now.
And indeed, if we look for callers of x86_supply_ssp, which supplies
the shadow stack pointer register, this is only called from amd64
specific code, either the native register fetching, or the core file
loading. There's no calls from i386 code.
And so, again, we have register number overlap, but we avoid any
issues by not making use of these registers for i386 linux.
Here's my question: Is this super clever design aimed at saving 12
bytes (3 * 4-byte registers) of space in the i386 regcache? Or is
this an accident where we happen to have gotten lucky?
If it's the first, then I really think there should be some comments
explaining what's going on.
If it's the second, then maybe we should fix this before it trips us
up?
This commit takes the second approach by doing the following:
1. In i386-tdep.h move all the *_NUM_REGS constants to be members of
'enum i386_regnum'. The I386_NUM_REGS value can be automatically
calculated based off the (current) last enum entry, and the
other *_NUM_REGS constants are calculated just as they previously
were, but are moved to keep them all together.
2. In i386-linux-tdep.h, I386_LINUX_ORIG_EAX_REGNUM and
I386_LINUX_NUM_REGS are moved into a new enum i386_linux_regnum,
the name of which is inspired by i386_regnum with the addition
of the linux tag. The first entry in this new enum starts from
I386_NUM_REGS rather than I386_PKRU_REGNUM. The
I386_LINUX_NUM_REGS will be calculated automatically by the
compiler.
3. In amd64-linux-nat.c, I extend amd64_linux_gregset32_reg_offset
so that it now has entries for the 3 registers that are no longer
aliasing, this stops an assert from the end of the file
triggering:
gdb_assert (ARRAY_SIZE (amd64_linux_gregset32_reg_offset)
== amd64_native_gregset32_num_regs);
As I386_LINUX_NUM_REGS has now increased by 3.
4. Given (3) I wondered why there was no assert being triggered from
the i386 code as i386_linux_gregset_reg_offset, in i386-linux-tdep.c
is clearly also wrong now.
So, In i386-linux-tdep.c I've added a new assertion at the end of
the file.
And then I've fixed i386_linux_gregset_reg_offset by adding the 3
new registers.
With these changes made I believe that the register number for the
$orig_eax register on i386 GNU/Linux targets should no longer be
aliasing with the SSP register.
For the reasons given above, I don't think this fixes any actual bugs,
it's more just a, lets not have unnecessary, and undocumented,
register number aliasing.
This change is visible using 'maint print registers', check out the
register number of $orig_eax before and after, it should now be +3
from where it was (changed from 72 to 75).
I did worry briefly about gdbservers that might not support XML target
descriptions and instead rely on a fixed GDB register numbering.
Though, if I'm honest, I have very little sympathy for such gdbservers
these days. Still, they could, potentially be tripped up by this
change. However, this is not the first time in recent years that the
value of I386_LINUX_ORIG_EAX_REGNUM has changed. This commit also
adjusted the register number:
commit 51547df62c
Date: Wed Feb 1 12:22:27 2017 +0100
Add support for Intel PKRU register to GDB and GDBserver.
And I'm not aware of any bug reports that came from this, we certainly
didn't feel the need to adjust the register number back again. So I'm
guessing that this renumbering will also go without issue.
Other than that, there should be no user visible changes after this
commit.
Reviewed-By: Christina Schimpe <christina.schimpe@intel.com>
README for GDB release
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try to use that version of Guile; or the file name of a
`pkg-config' executable, which will be queried to find the
information needed to compile and link against Guile.
`--enable-source-highlight'
When printing source code, use source highlighting. This requires
libsource-highlight to be installed and is enabled by default
if the library is found.
`--with-xxhash'
Use libxxhash for hashing. This has no user-visible effect but
speeds up various GDB operations such as symbol loading. Enabled
by default if libxxhash is found.
`--with-amd-dbgapi=[auto,yes,no]'
Whether to use the amd-dbgapi library to support local debugging of
AMD GCN architecture GPUs.
When explicitly requesting support for an AMD GCN architecture through
`--enable-targets' or `--target', there is no need to use
`--with-amd-dbgapi': `configure' will automatically look for the
amd-dbgapi library and fail if not found.
When using --enable-targets=all, support for the AMD GCN architecture will
only be included if the amd-dbgapi is found. `--with-amd-dbgapi=yes' can
be used to make it a failure if the amd-dbgapi library is not found.
`--with-amd-dbgapi=no' can be used to prevent looking for the amd-dbgapi
library altogether.
`--without-included-regex'
Don't use the regex library included with GDB (as part of the
libiberty library). This is the default on hosts with version 2
of the GNU C library.
`--with-sysroot=DIR'
Use DIR as the default system root directory for libraries whose
file names begin with `/lib' or `/usr/lib'. (The value of DIR
can be modified at run time by using the "set sysroot" command.)
If DIR is under the GDB configured prefix (set with `--prefix' or
`--exec-prefix' options), the default system root will be
automatically adjusted if and when GDB is moved to a different
location.
`--with-system-gdbinit=FILE'
Configure GDB to automatically load a system-wide init file.
FILE should be an absolute file name. If FILE is in a directory
under the configured prefix, and GDB is moved to another location
after being built, the location of the system-wide init file will
be adjusted accordingly.
`--with-system-gdbinit-dir=DIR'
Configure GDB to automatically load system-wide init files from
a directory. Files with extensions `.gdb', `.py' (if Python
support is enabled) and `.scm' (if Guile support is enabled) are
supported. DIR should be an absolute directory name. If DIR is
in a directory under the configured prefix, and GDB is moved to
another location after being built, the location of the system-
wide init directory will be adjusted accordingly.
`--enable-build-warnings'
When building the GDB sources, ask the compiler to warn about any
code which looks even vaguely suspicious. It passes many
different warning flags, depending on the exact version of the
compiler you are using.
`--enable-werror'
Treat compiler warnings as werrors. It adds the -Werror flag to
the compiler, which will fail the compilation if the compiler
outputs any warning messages.
`--enable-ubsan'
Enable the GCC undefined behavior sanitizer. By default this is
disabled in GDB releases, but enabled when building from git.
The undefined behavior sanitizer checks for C++ undefined
behavior. It has a performance cost, so if you are looking at
GDB's performance, you should disable it.
`--enable-unit-tests[=yes|no]'
Enable (i.e., include) support for unit tests when compiling GDB
and GDBServer. Note that if this option is not passed, GDB will
have selftests if it is a development build, and will *not* have
selftests if it is a non-development build.
`configure' accepts other options, for compatibility with configuring
other GNU tools recursively.
Remote debugging
=================
Remote debugging is when GDB runs on one machine type (see `--host'
above), while debugging applications running on a possibly different
machine using the 'target remote' or 'target extended-remote'
commands. The `--target' and `--enable-targets' configure options
control which targets GDB can debug. Remote debugging is one way in
which GDB can support cross-debugging. GDB runs on a machine of type
HOST (see `--host' configure option) but can remote debug a machine of
type TARGET (see `--target' and `--enable-targets' configure options).
The files m68k-stub.c, i386-stub.c, and sparc-stub.c are examples
of remote stubs to be used with remote.c. They are designed to run
standalone on an m68k, i386, or SPARC cpu and communicate properly
with the remote.c stub over a serial line.
The directory gdbserver/ contains `gdbserver', a program that
allows remote debugging for Unix applications. GDBserver is only
supported for some native configurations.
The file gdbserver/README includes further notes on GDBserver; in
particular, it explains how to build GDBserver for cross-debugging
(where GDBserver runs on the target machine, which is of a different
architecture than the host machine running GDB).
Reporting Bugs in GDB
=====================
There are several ways of reporting bugs in GDB. The preferred
method is to use the World Wide Web:
http://www.gnu.org/software/gdb/bugs/
As an alternative, the bug report can be submitted, via e-mail, to the
address "bug-gdb@gnu.org".
When submitting a bug, please include the GDB version number, and
how you configured it (e.g., "sun4" or "mach386 host,
i586-intel-synopsys target"). Since GDB supports so many
different configurations, it is important that you be precise about
this. The simplest way to do this is to include the output from these
commands:
% gdb --version
% gdb --config
For more information on how/whether to report bugs, see the
Reporting Bugs chapter of the GDB manual (gdb/doc/gdb.texinfo).
Graphical interface to GDB -- X Windows, MS Windows
==========================
Several graphical interfaces to GDB are available. You should
check:
https://sourceware.org/gdb/wiki/GDB%20Front%20Ends
for an up-to-date list.
Emacs users will very likely enjoy the Grand Unified Debugger mode;
try typing `M-x gdb RET'.
Writing Code for GDB
=====================
There is information about writing code for GDB in the file
`CONTRIBUTE' and at the website:
http://www.gnu.org/software/gdb/
in particular in the wiki.
If you are pondering writing anything but a short patch, especially
take note of the information about copyrights and copyright assignment.
It can take quite a while to get all the paperwork done, so
we encourage you to start that process as soon as you decide you are
planning to work on something, or at least well ahead of when you
think you will be ready to submit the patches.
GDB Testsuite
=============
Included with the GDB distribution is a DejaGNU based testsuite
that can either be used to test your newly built GDB, or for
regression testing a GDB with local modifications.
Running the testsuite requires the prior installation of DejaGNU,
which is generally available via ftp. The directory
ftp://sources.redhat.com/pub/dejagnu/ will contain a recent snapshot.
Once DejaGNU is installed, you can run the tests in one of the
following ways:
(1) cd gdb-VERSION
make check-gdb
or
(2) cd gdb-VERSION/gdb
make check
or
(3) cd gdb-VERSION/gdb/testsuite
make site.exp (builds the site specific file)
runtest -tool gdb GDB=../gdb (or GDB=<somepath> as appropriate)
When using a `make'-based method, you can use the Makefile variable
`RUNTESTFLAGS' to pass flags to `runtest', e.g.:
make RUNTESTFLAGS=--directory=gdb.cp check
If you use GNU make, you can use its `-j' option to run the testsuite
in parallel. This can greatly reduce the amount of time it takes for
the testsuite to run. In this case, if you set `RUNTESTFLAGS' then,
by default, the tests will be run serially even under `-j'. You can
override this and force a parallel run by setting the `make' variable
`FORCE_PARALLEL' to any non-empty value. Note that the parallel `make
check' assumes that you want to run the entire testsuite, so it is not
compatible with some dejagnu options, like `--directory'.
The last method gives you slightly more control in case of problems
with building one or more test executables or if you are using the
testsuite `standalone', without it being part of the GDB source tree.
See the DejaGNU documentation for further details.
Copyright and License Notices
=============================
Most files maintained by the GDB Project contain a copyright notice
as well as a license notice, usually at the start of the file.
To reduce the length of copyright notices, consecutive years in the
copyright notice can be combined into a single range. For instance,
the following list of copyright years...
1986, 1988, 1989, 1991-1993, 1999, 2000, 2007, 2008, 2009, 2010, 2011
... is abbreviated into:
1986, 1988-1989, 1991-1993, 1999-2000, 2007-2011
Every year of each range, inclusive, is a copyrightable year that
could be listed individually.
(this is for editing this file with GNU emacs)
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